In recent years, apparatuses have been widely used which compress and code an image by employing a coding method in which image information is treated as a digital value, and, at this time, the image information is compressed through orthogonal transform such as discrete cosine transform and motion compensation by using redundancy unique to the image information in order to transmit and accumulate information with high efficiency. The coding method includes, for example, Moving Picture Experts Group (MPEG) or the like.
Particularly, MPEG2 (ISO/IEC 13818-2) is defined as a general use image coding method, and is a standard covering both an interlaced scanning image and a progressive scanning image, and a standard resolution image and a high definition image. For example, MPEG2 is currently widely used in extensive applications for professional use and consumer use. By the use of the MPEG2 compression method, it is possible to assign a bit rate of 4 Mbps to 8 Mbps to an interlaced scanning image of a standard resolution having 720×480 pixels, for example. In addition, by the use of MPEG2, it is possible to assign a bit rate of 18 Mbps to 22 Mbps to an interlaced scanning image of a high resolution having 1920×1088 pixels, for example. Accordingly, it is possible to realize a high compression ratio and good image quality.
MPEG2 has mainly targeted high image quality coding suitable for broadcasting, but has not handled a coding method at a bit rate lower than that in MPEG1, that is, at a higher compression ratio. With the wide use of portable terminals, the need for such a coding method has been considered to increase, and thus a MPEG4 coding method has been standardized so as to correspond thereto. In relation to an image coding method, a standard thereof was approved as an international standard entitled ISO/IEC 14496-2 on December, 1998.
As for the standardization schedule thereof, the coding method has become an international standard under the name of H.264 and MPEG-4 Part 10 (Advanced Video Coding which will be hereinafter referred to as AVC) in March 2003.
In addition, as an extension of the AVC method, Fidelity Range Extension (FRExt) which includes coding tools for use in business such as RGB or 4:2:2 and 4:4:4 and also includes 8×8 DCT or quantization matrix defined in MPEG2 was standardized in February, 2005. This realizes a coding method in which even film noise included in a movie can be favorably expressed by using the AVC method, and thus leads to use for various applications such as Blu-Ray Disc (registered trademark).
However, recently, there have been increasing need for higher compression ratio coding, such as a need for compression of an image with about 4000×2000 pixels which is four times the size of a high-vision image or a need for delivery of a high-vision image in limited transmission capacity circumstances such as the Internet. For this reason, study of improvement of coding efficiency is being currently performed in Video Coding Expert Group (VCEG) affiliated to the above ITU-T.
As one of methods for improving coding efficiency, a method (hereinafter, referred to as MV competition) has been proposed in which, in order to improve coding of a motion vector using median prediction in the AVC method, either “temporal predictor” or “spatio-temporal predictor” is adaptively used as prediction motion vector information in addition to “spatial predictor” which is defined in the AVC method and is obtained through the median prediction (for example, refer to NPL 1).
In addition, in the AVC method, a cost function value in a high complexity mode or a low complexity mode implemented in reference software using the AVC method, called Joint Model (JM), is used to select prediction motion vector information.
In other words, a cost function value when prediction motion vector information is used is selected, and optimal prediction motion vector information is selected. Flag information indicating information regarding which prediction motion vector information is used in each block is transmitted in image compression information.
However, there is a concern that a macroblock size of 16 pixels×16 pixels may not be optimal for a large picture frame such as Ultra High Definition (UHD; 4000 pixels×2000 pixels) which will become a target of a next-generation coding method.
Therefore, currently, for the purpose of improvement in higher coding efficiency than that of AVC, standardization of a coding method called High Efficiency Video Coding (HEVC) is in progress by Joint Collaboration Team-Video Coding (JCTVC) which is a joint standardization organization of International Telecommunication Union Telecommunication Standardization Sector (ITU-T) and International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC) (for example, refer to NPL 2).
In the HEVC coding method, a coding unit (CU) which is the same process unit as the macroblock in the AVC method is defined. The CU is not fixed in a size of 16×16 pixels, unlike in the macroblock of the AVC method, and is designated in image compression information for each sequence. In addition, in each sequence, the largest coding unit (LCU) and the smallest coding unit (SCU) of the CU are prescribed.
Further, as one of the coding methods of motion information, a method (hereinafter, referred to as a merge mode) called motion partition merging has been proposed (for example, refer to NPL 3). In this method, in a case where motion information of a corresponding block is the same as motion information of a neighboring block, only flag information is transmitted, and, during decoding, the motion information of the corresponding block is reconstructed by using the motion information of the neighboring block.
In other words, also in the merge mode, a spatial predictor (spatial prediction motion vector) and a temporal predictor (temporal prediction motion vector) are obtained from the neighboring blocks, and an optimal prediction motion vector is determined from thereamong. Further, in the merge mode, in a case where the determined prediction motion vector and motion information of a corresponding block are the same as each other, only flag information is transmitted.
Meanwhile, a spatial predictor of a corresponding PU which is a process target is obtained in a motion vector coding or decoding process in the above MV competition or merge mode. In this case, a method has been proposed in which a motion vector of a PU adjacent to the corresponding PU in a predetermined positional relationship among PUs adjacent thereto is used as a candidate of a spatial predictor of the corresponding PU.
Specifically, a motion vector of A0 which is a PU adjacent to the lower left side of the corresponding PU, and a motion vector of A1 which is a PU located on A0 among PUs adjacent to the left side of the corresponding PU, are used as candidates. In addition, a motion vector of B2 which is a PU adjacent to the upper left side the corresponding PU, a motion vector of B0 which is a PU adjacent to the upper right side of the corresponding PU, and a motion vector of B1 which is a PU located on the left side of B0 among PUs adjacent to the upper side of the corresponding PU, are used as candidates.
In addition, scanning is performed in an order of A0 and A1 and an order of B0, B1 and B2, and the scanning finishes at the time when motion vector information having a reference frame which is equivalent to motion vector information of the corresponding PU is detected.